The present invention relates to apparatus for cutting or sealing plastics materials, in particular plastics packaging materials in sheet, roll or bag form.
It is known to seal or join flexible plastics packaging material by welding using heat supplied for example by electrically heated resistance wires, or by heated metal dies (cartridge heaters). The electrically heated resistance wires become extremely hot in order to effect sealing and localized overheating and even fuming of the plastics material is not uncommon. In contrast, the temperature of the cartridge heaters is more closely controlled since too low a temperature will result in poor or no sealing of the plastics film, whereas too high a temperature would seriously damage, the plastic film again resulting in poor or no sealing. Historically, cartridge heaters have comprised quite large metal blocks having a high thermal inertia in an attempt to provide temperature stability. This however means that it is impossible quickly to change or even correct the die temperature relative to a required target temperature. With both electrically heated resistance wires and cartridge heaters, cutting of the plastics film is sometimes achieved with a separate cutting operation.
The present applicant has pioneered so-called low thermal inertia dies for cutting or sealing plastics material. The low thermal inertia dies do not rely on thermal inertia to maintain desired temperature and resist temperature fluctuations (as distinct from conventional cartridge heaters), and do not rely on heating to temperatures well in excess of that required to seal plastics materials. Instead, a low thermal inertia die is heated to an exact temperature required to seal the plastics material in question, and as heat energy is dissipated during operation it is replaced by an equivalent amount of thermal energy from an electrical heating element mounted on the die.
When using a low thermal inertia die, accurate control of the temperature of the die is essential. In high throughput applications in particular, the desired die temperature must be restored as quickly as possible during each sealing operation in readiness for the next plastics material for sealing. A need is seen for an improved low thermal inertia sealing die which enables even more accurate sealing operations to be performed.
In accordance with a first aspect of the present invention, there is provided apparatus for cutting or sealing plastics material, comprising a low thermal inertia die having a working face which in use opposes plastics material to be cut or sealed, and an electrical resistance heating element for heating the low thermal inertia die, wherein the electrical resistance heating element is embedded in the low thermal inertia die.
Embedding the electrical resistance heating element in the low thermal inertia die enables highly efficient transfer of heat energy from the element to the die. Embedding also enables the electrical resistance heating element to be positioned substantially adjacent the working face of the electrical resistance heating element, in which case heat energy will be provided where the majority will be dissipated. Such an arrangement offers improved performance over a prior low thermal inertia die where the heating element was located on the surface of the die, remote from the working face. As heat energy is delivered exactly where it is required, low voltage heating elements may be used.
The working face of the low thermal inertia die may have a profile adapted to achieve predetermined sealing or cutting geometry. The profile may include a ridged region to provide an edge for cutting and producing narrow seal bands either side of the cut. The profile may include a chamfered region, perhaps either side of the ridged region, to provide an enlarged surface for producing wider seal bands.
The apparatus may further comprise means for controlling electrical current supplied to the electrical resistance heating element. This control may be effected in response to the temperature of the low thermal inertia die. precise temperature control is extremely important with low thermal inertia dies, which typically will operate from 140-220xc2x0 C., depending upon the plastics material being sealed/cut.
The apparatus may further comprise means for sensing the temperature of the working face of the low thermal inertia die. The temperature sensing means may comprise a thermocouple, with the tip of the thermocouple located inside the low thermal inertia die, adjacent the working face. Preferably, the thermocouple is located to sense the temperature of a central region of the work face. Alternatively, the temperature sensing means may comprise means for measuring electrical resistance of the electrical resistance heating element, and means for determining temperature in dependence upon electrical resistance measured.
The electrical resistance heating element may be elongate, and may be received in a groove provided in the low thermal inertia die. At least one side or floor of the groove is preferably adjacent the working face. The electrical heating element may comprise a low resistance nickel-chrome strip covered in an electrical insulator. Preferably, the electrical insulator is thermally conductive. The electrical heating element may be encased within a metal housing, with the electrical insulator isolating the electrical heating element from the metal housing. The metal housing enables heat to be efficiently drawn from the electrical heating element. The metal housing may be formed from a tube. Extensive contact between the metal tube and the electrical insulator may be achieved by flattening the metal tube around the electrical heating element. The metal housing may have a profile to be a snug fit in the groove. Snug or friction fitting between the metal housing and the low thermal inertia die enables good thermal contact there between. Additional thermal transfer may be achieved using a conductive filler or bonding agent such as silicone.
The apparatus may further comprise a bolster for supporting the low thermal inertia die, and thermally insulated holding means for mounting the low thermal inertia die on the bolster. The thermally insulated holding means prevents significant amounts of heat energy escaping to the bolster which could otherwise represent a massive heat sink. The apparatus may further comprise an anvil, with at least one of the low thermal inertia die and the anvil being moveable towards the other for biasing the work face against opposed plastics material. The anvil may be electrically heated. Heating the anvil may increase running speeds of the apparatus and may improve feed quality. The temperature of the anvil may be controlled in response to the output of a temperature sensing means associated there with. The surface of the anvil facing the working face may be planar, or it may have a profile corresponding to that of the working face of the low thermal inertia die. The anvil may even be of rotary type, perhaps rotating at a speed to match movement of plastics material past the die. The anvil may be of rubber, silicon, PTFE or fibre glass.
The working face of the low thermal inertia die may be covered with a non-stick coating. The low thermal inertia die may be of aluminium, and may be formed by extrusion, possible with the groove for receiving the electrical resistance heating element. The working face of the low thermal inertia die may be linear or curvilinear, even circular. With a low thermal inertia die, the mass of material constituting the die itself is usually low. Accordingly, the low thermal inertia die may be blade-like, with the depth of the die being greater than the width of the die. In order to be adjacent the working face or edge of a blade-like die, a strip-shaped electrical resistance heating element may be embedded in the low thermal inertia die with one elongate edge closer to the working face than the other.
Desirably arms or clamp side plates may be provided to either side of the low thermal inertia die for contacting the plastics sheeting, carrying it into contact with the anvil and holding it in position as it is cut or sealed.
The arms are preferably carried on the bolster by resilient members allowing relative movement of the arms relative to the bolster. Alternatively the arms may be fixed relative to the bolster.
Control of the temperature of the low thermal inertia die is preferably effected in response to the output of the temperature sensing means by a control unit which controls operation of a relay in the power circuit of the electrical resistance heating element.
The control unit may be programmed/programmable to control the heating effect of the electrical current sent to the electrical resistance heating element. Parameters which may be programmed/programmable within the controller include the length of time for which the low thermal inertia die must be heated before it reaches an operating temperature; the temperature at which it is desired to operate the low thermal inertia diexe2x80x94the set point; and the range of temperatures to either side of the set point temperature which is permissible; and a maximum set point temperature above which the low thermal inertia die cannot be heated. The set point is dependent upon a number of factors, including thickness and type of plastics material and desired speed of operation.
In accordance with another aspect, the invention provides apparatus for cutting and/or sealing sheet or roll plastics materials comprising a blade which is electrically heated and which is movable to bias the plastics sheet or roll against an anvil, wherein the blade is supported in a bolster by thermally insulating holders.
The bolster may be movable to carry the blade to bias the plastics sheet or roll against the anvil; or the anvil may be movable to bias the plastics sheet or roll against the heated blade; or both bolster and anvil may be movable.